1. Field of the Invention
The present invention relates to a radio receiving apparatus used in mobile communications equipment, and more particularly to a radio receiving apparatus which can reduce the number of high-frequency circuit parts, and hence can reduce factors of high power consumption and factors causing unstable operation which factors are present in high-frequency circuits.
The invention also relates to communications equipment which transmits information while changing the bandwidth in accordance with its type, and more particularly it is intended to implement, in a simple configuration, a radio receiving apparatus for receiving communication signals of different bandwidths.
2. Description of the Related Art
One of the important points of a radio receiving apparatus in mobile communications equipment is how to reduce the number of high-frequency circuit parts to reduce factors of high power consumption and factors causing unstable operation, which are present in high-frequency circuits, thereby reducing the manufacturing cost and a space occupied by the high-frequency circuit parts. One of the causes for the fact that the high-frequency parts of the radio receiving apparatus are made complex is that it is very difficult to realize a sharply-attenuating channel filter for separating a desired channel band from its adjacent channels, and a desired filtering characteristic needs to be established step by step.
An example of the configuration of a radio receiving scheme used in current mobile communications equipment is shown in FIG. 38. In addition, as another conventional example, FIG. 39 shows a direct demodulation scheme in which the local oscillation frequency is set at a carrier frequency, i.e., a direct-conversion receiving scheme for direct conversion into a baseband (Japanese Unexamined Patent Publication No. Hei. 6-164243).
In FIG. 38, a radio signal having a frequency fc enters an antenna ANT, and is amplified by a low-noise amplifier LNA. The amplified radio signal is passed through a bandpass filter BPF1 to separate overall subject frequency channels of the communications system from other communication signal groups. Its output is converted into a first intermediate frequency by a frequency converter MIX1, and signal components other than the desired frequency channel are eliminated by a first intermediate frequency filter IF1-FLT as much as possible. Its output is amplified by a first intermediate frequency amplifier IF1-AMP, and is then supplied to a frequency converter MIX2.
As for the received signal which has been converted into a second intermediate frequency, signal components other than the desired frequency channel are further eliminated by a second intermediate frequency amplifier IF2-FLT. Its output is amplified by a second intermediate frequency amplifier IF2-AMP, and is then inputted to a quadrature wave detector Q-DET.
Here, the received signal is subjected also to frequency conversion with a second intermediate frequency fL0, into the baseband. The received signal is passed through a lowpass filter LPF to eliminate the signal components other than the frequency channel and to eliminate image signals in frequency conversion. Thus the desired channel signal is extracted, and is amplified to a predetermined signal strength by a baseband amplifier BF-AMP, thereby providing a reception output.
Accordingly, a description will be first given of problems encountered in the radio receiving apparatus of communications equipment which is used in the vicinity of a microwave band and which is shown in FIG. 38 illustrating a conventional example.
As a first problem, as seen in the conventional example in FIG. 38, frequency conversion in three stages is carried out including quadrature detection, and filtering in four stages and amplification in four stages are effected. Three local oscillators of L01, L02, and fL0 are needed. Therefore, the radio receiving apparatus requires numerous parts.
A second problem is that these numerous parts result in large power consumption.
Next, a consideration will be made of the example of FIG. 39, i.e., the direct-conversion receiving apparatus in which an attempt is made to simplify the radio receiving apparatus. In FIG. 39, a received AM high-frequency signal is inputted to a pair of mixers 18 and 19 and mixed with respective high-frequency signals whose frequency is equal to the carrier frequency and phases are different from each other by 90.degree..
Outputs of the mixers 18 and 19 are respectively inputted to phase shifters 27 and 28 via lowpass filters 23 and 24 and A/D converters 25 and 26. The respective signals whose phases are delayed in such a manner as to be mutually different by 90.degree. by the phase shifters 27 and 28 are inputted to a matrix circuit 29 where signals representing the sum of and the difference between the respective signals are derived.
The signals from the matrix circuit 29 are converted to analog signals by D/A converters 30 and 31, modulated signals in both sidebands of the AM high-frequency signal are separated, and a signal with less noise is selectively outputted to a speaker 35. A direct-conversion receiving apparatus which has less noise and radio interference is realized.
Consideration will be given to the power consumption of the circuits and the performance required of the parts in this conventional example. In the conventional example of FIG. 39, the channel filter for separating and extracting the received signal from adjacent interfering signals is implemented by the lowpass filters 23 and 24 and digital filters provided in digital circuits after A/D conversion.
Where signal processing is carried out by digital circuits in a demodulation circuit 42, it is possible to simplify the filters 23 and 24 for the radio system. However, in order to obtain sufficient amplitude-separating capabilities and frequency-separating capabilities, the calculation clock rate must be sufficiently higher than the highest frequency component of the analog signal. Hence, since the operating speed of the operating portion becomes high and the operating amplitude in the digital system 42 is fixed and large at several volts, there is a drawback in that this results in an increase in power consumption which is several times greater than in a case where processing is effected by an analog system.
Further, many processing systems operate in parallel in logical circuits. That is, even if the calculation clock rate is close to a baseband frequency, the total power consumption of the circuitry becomes (square of the voltage amplitude).times.(processing speed).times.(electrostatic capacitance of the circuit system load).times.(number of parallels), so that the total power consumption becomes large. Namely, the fact that the signal is processed by a digital circuit has a negative factor for increasing the power consumption.
As a third problem, in a case where an attempt is made to digitize the signal processing, there results an increase in power consumption which is several times greater than in a case where processing is effected by a radio system.
As a fourth problem, conventional digital filters involve complicated calculations, and require the four rules of arithmetic even when their configurations are simple, so that power consumption is not negligible.
In addition, if consideration is given to the A/D converters 25 and 26 for digitizing the signal, the voltage amplitude required of the input signal is generally large at one volt or two volts. Accordingly, in the conventional example shown in FIG. 39, the abilities to supply the amplitude are required of the mixers 18 and 19 in the preceding stage. It may be said that this is possible in frequencies of AM radio bands, i.e., medium-wave broadcasting bands, which are handled by the conventional example of FIG. 39; however, in higher frequency bands such as those for TV broadcasting and cellular phone systems, no mixers are available which are capable of obtaining such large outputs. For this reason, it is generally necessary to amplify a voltage by inserting amplifiers in a stage preceding the A/D converters. Therefore, as a fifth problem, if a method using the A/D converters is adopted, there is a negative factor for substantially increasing the power consumption in the radio system or the analog system.
In addition, in the conventional scheme shown in FIG. 39, the frequency generated by the local oscillator is equal to the carrier frequency of the received signal. For this reason, as a sixth problem, trouble occurs in many communication schemes. Namely, as shown in FIG. 40(a), since the oscillation frequency is identical to a harmonic frequency of the reception circuit system, the oscillation frequency leaks to the reception circuit system, so that the leaked oscillation frequency interferes adjacent stations (cellular phones) from the antenna or enters from the received-signal input sides of the mixers 18 and 19. In the mixers 18 and 19, the mixing, i.e., the multiplication, of local oscillation signals occurs, and a DC component occurs as shown in FIG. 40(b) (the center of the phase circle on the I-Q plane deviates), with a result that the DC component imparts errors to a demodulated signal in the form of a DC offset component (for instance, an increase in BER). Accordingly, the direct-conversion receiving scheme of the type which selects the carrier frequency as the local oscillation frequency has been mostly adopted in communications using frequency demodulation schemes which are relatively resistant against single-frequency interference.
Here, the above-described problems will be summed up below.
The first problem is that the radio receiving apparatus requires numerous parts for assuring favorable reception channel selectivity.
The second problem is that the numerous parts which present the aforementioned first problem incur large power consumption.
The third problem is that digitization of signal processing requires power consumption which is several times greater than analog processing.
The fourth problem is that the conventional digital filters involve complex calculations, so that their power consumption is large.
The fifth problem is that the A/D converters for signal digitization require large input signal amplitudes.
The sixth problem is that, in the direct-conversion receiving scheme whose local oscillation signal is equivalent to the carrier frequency of the received signal, the local oscillation signal interferes adjacent stations from the antenna, and a DC offset occurs, imparting errors to the demodulated signal.
Now, turning to the issue of a system generally called a multi-band system, to receive communication signals of different bandwidths, it is conventionally necessary to use channel filters in the same number as the number of the types of bandwidths. This increases the size of a radio receiving apparatus and the addition of a circuit for switching between the filter groups prevents further reductions in size and power consumption.
FIG. 54 shows the main part of a conventional radio receiving apparatus for receiving multi-band signals which is disclosed in Japanese Unexamined Utility Model Publication No. Sho. 62-171228. Signals received by an antenna 101 are supplied via a high-frequency amplifier 103 to four filters which, being switched by switches 104A and 104B, extracts a desired frequency-band component. The extracted signal is supplied to two demodulation circuits 115 and 116 via a frequency converter 113 which also receives a local oscillation signal 109 for frequency conversion. In this conventional apparatus, the demodulation circuits 115 and 116 are for FM and AM signals having different bandwidths. To accommodate such different band widths, the intermediate frequency amplification stages of the respective demodulation circuits 115 and 116 have different intermediate frequency filters. Therefore, six or more filters are needed in total.
FIGS. 55(a)-55(d) show examples of frequency utilization in mobile multi-band communications systems for which extensive studies have been made in recent years. In current cellular phone systems, narrow frequency bandwidth channels are arranged at regular frequency intervals as shown in FIG. 55(d). In the case of accommodating not only telephone (voice signals) but also data communication which involves a larger amount of information, a broad bandwidth is formed by combining a plurality of channels as shown in FIGS. 55(c) and 55(b). To transmit an even larger amount of information of moving pictures, for instance, there may occur a case where a broad bandwidth is formed by combining all the communication channels of a given communication band as shown in FIG. 55(a).
In a communication system that utilizes many kinds of bandwidths, if filters are so prepared as to correspond to the respective bandwidths as in the above conventional examples, serious problems will occur in application to portable equipment and the like, such as an increase in size, increased complexity in shape, need for adding switches to peripheral sections, and an increase in power consumption due to parasitic capacitances.